Stainless steel for polymer fuel cell separator and method of manufacturing the stainless steel

ABSTRACT

Provided are stainless steel for a polymer fuel cell separator and a method of manufacturing the stainless steel, in which a surface modification technique based on wet processing is applied to a surface of stainless steel used for parts such as an anode and a cathode, etc., of a stack that generates electricity, thereby improving corrosion resistance and electric conductivity, and preventing moisture from being formed.

TECHNICAL FIELD

The present disclosure relates to stainless steel for a polymer fuelcell separator used in an electricity generator using a hydrogen fuel,and a method of manufacturing the stainless steel, and moreparticularly, to stainless steel for a polymer fuel cell separator and amethod of manufacturing the stainless steel, in which a surfacemodification technique based on wet processing is applied to a surfaceof stainless steel used for an anode, a cathode, etc., of a stack thatgenerates electricity, thus providing corrosion resistance andelectrical conductivity and preventing moisture from being formed.

The present disclosure also relates to stainless steel for a polymerfuel cell separator and a method of manufacturing the stainless steel,in which durability is superior with electrical, chemical, and physicalstability by using a surface modification technique based on a chemicalmethod instead of a coating method or a passivation processing method.

BACKGROUND ART

A hydrogen fuel cell is an eco-friendly electricity generator thatgenerates water and electrical energy by using hydrogen and oxygen inthe atmosphere, reversely to a method of generating hydrogen and oxygenbased on water electrolysis with an eco-friendly electric powergeneration system that has emerged as a scheme to replace a fossil fueland reduce air pollution. Among fuel cells, especially, a polymer fuelcell (a proton exchange membrane fuel cell: PEMFC) has advantages suchas a low operating temperature of 60 to 80° C., a high energyefficiency, etc.

The polymer fuel cell includes a polymer electrolyte membrane, anelectrode, a gas diffusion layer (GDL), and a separator. The separatorused in the polymer fuel cell has a structure having formed therein achannel through which hydrogen and oxygen may flow, and separates eachunit cell, serves as a support of a membrane electrode assembly (MEA),provides a path through which hydrogen and oxygen may flow, and plays animportant role as a current collector that delivers generated energy.

In the polymer fuel cell (PEMFC) generating electricity by using ahydrogen fuel, hydrogen supplied from a fuel electrode (an anode) isdissociated into hydrogen ions and electrons while passing through anelectrolyte layer, and the hydrogen ions move to an air electrode (acathode) and react with oxygen to form water, and the electrons movefrom the electrolyte layer to an electricity storage device (a battery)through an external circuit to form electricity, such that electricityis produced based on an electrochemical reaction between hydrogen andoxygen.

The polymer fuel cell includes a polymer electrolyte membrane, anelectrode, a GDL, and a separator. The separator used in the polymerfuel cell has a structure having formed therein a channel through whichhydrogen and oxygen may flow, and separates each unit cell, serves as asupport of a membrane electrode assembly (MEA), provides a path throughwhich hydrogen and oxygen may flow, and plays an important role of acurrent collector that delivers generated energy.

A fuel cell separator, which refers to a fuel electrode (an anode) andan air electrode (a cathode), enables an electrochemical reactionbetween hydrogen and oxygen. Thus, the fuel cell separator requiressuperior electrical conductivity to facilitate movement of electrons andneeds to guarantee corrosion resistance to prevent corrosion in areaction between the anode and the cathode. To prevent moisturecondensation (a flooding phenomenon) by lowering a surface tension forsmoothly discharging water generated by an electrochemical reactionbetween hydrogen and oxygen, excellent hydrophilicity is required.

The fuel cell separator requires superior thermal and electricalconductivity, a mechanical strength for preventing deformation andavoiding destruction due to vibration or shocks, dimensional stability,chemical resistance, etc., and graphite, a Ti alloy, conductive plastic,stainless steel, etc., have such properties, and as a metal electrodeplate, stainless steel is used for excellent thermal conductivity,formability, dimensional stability, and corrosion resistance, andconductivity is given to the electrode plate by coating the electrodeplate or forming a conductive film on the electrode plate.

A carbon (or graphite)-based separator has a high gas or liquidpermeability and poor mechanical strength and moldability, and has ahigh processing cost. On the other hand, a stainless metal separator hassuperior gas tightness and high thermal and electrical conductivity, andis able to be thinned, guaranteeing lightweightness and excellent shockresistance. Moreover, the stainless metal separator has the advantage ofreducing the price of the separator with excellent production yield inapplication of a thin plate forming process such as stamping,hydroforming, etc.

With this advantage, a demand for using stainless steel as a polymerfuel cell separator has been increasing recently. Stainless steel has anexcellent thinning property and formability, but has a high contactresistance due to a Cr₂O₃ passivation film formed on a surface of thestainless steel, resulting in low electrical conductivity.

To solve this problem, after a separator is manufactured by forming aflow path in stainless steel, a conductive material, such as Au, Pt, C,etc., is coated onto a surface of the fuel cell separator to giveconductivity in a final process, thereby improving electricalconductivity.

However, a process of coating the conductive material such as Au, Pt, C,etc., occupies a high proportion of a manufacturing cost of the fuelcell separator, such that this issue needs to be solved to improveproduction cost and yield.

Stainless steel is made of an alloy including iron and chromium orincluding iron, chromium, and nickel, and the corrosion resistance ofthe stainless steel may be excellent by forming a very thin chrome oxidelayer (Cr₂O₃) in which chromium (Cr) is combined with the surface of thesteel to block intrusion of oxygen into a metal matrix. As the chromeoxide layer (Cr₂O₃) formed on the surface of stainless steel is formedfirmly, corrosion resistance may be superior, but as the firm oxidelayer is formed, electrical conductivity may be degraded.

With such properties, for the separator, a method of coating or carboncoating using gold or white gold, which is precious metal with excellentcorrosion resistance and electrical conductivity, and a method offorming a conductive film after stainless passivation are applied.

A related prior document, Korean Patent Publication Gazette No.10-2015-0074768 (published on Jul. 2, 2015) discloses austenite-basedstainless steel for a fuel cell and a method of manufacturing thestainless steel.

DETAILED DESCRIPTION OF INVENTION Technical Problem

The present disclosure provides stainless steel for a polymer fuel cellseparator and a method of manufacturing the stainless steel, in which achemical surface modification technique, instead of coating orconductive film formation, is applied to simultaneously implementcorrosion resistance and electrical conductivity that are mechanicalproperties for use as a fuel cell separator, thereby securing a superiorinterfacial contact resistance.

The present disclosure also provides stainless steel for a polymer fuelcell separator and a method of manufacturing the stainless steel, inwhich a thermal oxide film generated in a steel making and rollingprocess or a natural oxide film formed in an atmosphere environment areremoved by etching and a chemical surface modification technique isapplied, such that the stainless steel has a contact resistance of 10 to35 mΩ·cm² under a pressure of 1.0 MPa, and has a contact angle of30°˜70° in a contact angle (water droplet formation) test to provide ahigh-temperature potentiodynamic corrosion resistance of 15 μA/cm² orless and remove moisture condensation in a mixed solution of sulfuricacid of 0.1 N and hydrofluoric acid of 2 ppm at a temperature of 80° C.that is a polymer fuel cell environment temperature.

Solution to Problem

According to an aspect of the present disclosure, stainless steel for apolymer fuel cell separator includes, by weight %, C: 0.01˜0.08%, Si:0.3˜1.0%, Mn: 0.3˜2.0%, Cr: 15˜35%, Cu: 1.0% or less, N: 0.01˜0.05%, Ti:0.3% or less, Nb: 0.3% or less, and as the rest, Fe and other inevitableimpurities, and a surface modification layer having an interfacialcontact resistance of 10˜35 mΩcm² through surface modificationprocessing, in which a potentiodynamic corrosion resistance is 1.0˜15.0μA/cm² and a contact angle is 30˜70°.

The stainless steel may further include one or more kinds of P: 0.14weight % or less, S: 0.03 weight % or less, H: 0.004 weight % or less,and O: 0.007 weight % or less.

The surface modification processing may include degreasing processing ofperforming immersion in a degreasing solution, etching and desmutprocessing of performing etching with an etching solution and desmutprocessing in a desmut solution, and surface stabilization processing ofperforming immersion in a surface stabilization solution.

According to another aspect of the present disclosure, a method ofmanufacturing stainless steel for a polymer fuel cell separatorincludes, (a) providing a stainless base material including, by weight%, C: 0.01˜0.08%, Si: 0.3˜1.0%, Mn: 0.3˜2.0%, Cr: 15˜35%, Cu: 1.0% orless, Ni: 10˜25%, N: 0.01˜0.05%, Ti: 0.3% or less, Nb: 0.3% or less, andas the rest, Fe and other inevitable impurities, the stainless basematerial having a passivation film formed on a surface thereof, (b)performing degreasing processing by immersing the surface of thestainless base material in a degreasing solution, (c) after etching thedegreasing-processed stainless base material with an etching solution,performing desmut processing with a desmut solution, and (d) performingsurface stabilization on the etched and desmut-processed stainless basematerial with a surface stabilization solution, in which after (d), thestainless steel includes a surface modification layer having aninterfacial contact resistance of 10˜35 mΩ·cm², through surfacemodification processing.

The stainless base material may further include one or more kinds of P:0.14 weight % or less, S: 0.03 weight % or less, H: 0.004 weight % orless, and O: 0.007 weight % or less.

In (b), the degreasing processing may include performing immersion inthe degreasing solution at 30˜70° C. for 0.5˜5 minutes.

In (c), the etching processing may include performing immersion for0.2˜2 minutes in an etching solution, heated to 40˜80° C., including atleast two or more of 2.5˜6.2 mols of sulfate ions, 0.1˜2.0 mols ofnitrate ions, and 1.0˜5.0 mols of fluorine, and the desmut processingmay include performing immersion for 0.5 2 minutes in a desmut solution,heated to 40˜80° C., including 1.5˜6.0 mols of hydrogen peroxide,1.0˜4.0 mols of fluorine, and 0.001˜0.01 mol of a corrosion inhibitor.

After (d), the method may further include (e) performing stabilizationthermal processing for stabilization of the surface-stabilized stainlessbase material.

The stabilization thermal processing may be performed at 150˜250° C. for1˜10 minutes.

Advantageous Effects of Invention

Stainless steel for a polymer fuel cell separator and a method ofmanufacturing the stainless steel according to the present disclosuremodify a surface passivation film without corrosion or pitting withrespect to a stainless base material having a passivation film on asurface thereof, thereby securing a low interfacial contact resistance,superior corrosion resistance, and excellent wettability.

Moreover, the stainless steel for a polymer fuel cell separator and themethod of manufacturing the stainless steel according to the presentdisclosure may provide a superior interfacial contact resistance andcorrosion resistance without additional coating of precious metal orconductive materials, and provide excellent surface wettability withoutfurther hydrophilic processing or coating.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart illustrating a method of manufacturing stainlesssteel for a polymer fuel cell separator, according to an embodiment ofthe present disclosure.

FIG. 2 is a schematic diagram for describing a process of measuringinterfacial contact resistances of samples according to Embodiments 1through 6 and Comparative Examples 1 through 9.

FIG. 3 is a graph showing potentiodynamic corrosion resistancemeasurement results with respect to samples manufactured according toComparative Examples 1 through 3 and 5.

FIG. 4 is a graph showing potentiodynamic corrosion resistancemeasurement results with respect to samples manufactured according toEmbodiments 1 through 4.

FIG. 5 shows pictures of contact angles of samples manufacturedaccording to Comparative Examples 1 through 3 and 5.

FIG. 6 shows pictures of contact angles of samples manufacturedaccording to Embodiments 1 through 4.

FIG. 7 shows pictures indicating whether a corrosion product (smut)remains for samples manufactured according to Embodiments 3 and 5 andComparative Example 3.

FIG. 8 is a graph showing X-ray photoelectron spectroscopy (XPS)analysis results with respect to samples manufactured according toEmbodiments 1 and 3 and Comparative Example 1.

FIG. 9 is a graph showing stack performance measurement results withrespect to a unit cell using samples manufactured according toEmbodiments 3 and 6 and Comparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Advantages and features of the present disclosure and a method forachieving them will be apparent with reference to embodiments describedin detail, together with the attached drawings. However, the presentdisclosure is not limited to the disclosed embodiments, but may beimplemented in various manners, the embodiments are provided to completethe disclosure of the present disclosure and to allow those of ordinaryskill in the art to understand the category of the present disclosure,and the present disclosure is defined by the category of the claims.Throughout the specification, an identical reference numeral willindicate an identical component.

Hereinafter, with reference to the accompanying drawings, stainlesssteel for a polymer fuel cell separator and a method of manufacturingthe stainless steel according to an embodiment of the present disclosurewill be described in detail.

Stainless steel for a polymer fuel cell separator according to anembodiment of the present disclosure may include, by weight %, C:0.01˜0.08%, Si: 0.3˜1.0%, Mn: 0.3˜2.0%, Cr: 15˜35%, Cu: 1.0% or less, N:0.01˜0.05%, Ti: 0.3% or less, Nb: 0.3% or less, and as the rest, Fe andother inevitable impurities, and may include a surface modificationlayer having an interfacial contact resistance of 10˜35 mΩ·cm² under apressure of 1.0 MPa through surface modification processing.

The stainless steel for the polymer fuel cell separator according to anembodiment of the present disclosure may further include one or morekinds of P: 0.14 weight % or less, S: 0.03 weight % or less, H: 0.004weight % or less, and O: 0.007 weight % or less.

As a result, the stainless steel for the polymer fuel cell separatoraccording to an embodiment of the present disclosure may have apotentiodynamic corrosion resistance of 15 μA/cm² or less (sulfuric acidof 0.1 N and hydrofluoric acid of 2 ppm at 80° C., SCE), morepreferably, 1.0˜15.0 μA/cm², and a contact angle of 70° or less, morepreferably, a contact angle range of 30°˜70°, when a droplet of 3 μl isdropped.

Hereinbelow, a role and a content of each component included in thestainless steel for the polymer fuel cell separator according to anembodiment of the present disclosure will be described.

Carbon (C)

Carbon (C) is an essential element for increasing the strength ofaustenitic stainless steel, and may be preferably added at a contentratio of 0.01˜0.08 weight % to a total weight. When the amount ofaddition of carbon (C) is less than 0.01 weight %, a refining price formaking a high-purity product may increase. In contrast, when the amountof addition of carbon (C) exceeds 0.08 weight %, processability andtoughness may be degraded due to increase of impurities.

Silicon (Si)

Silicon (Si) is an effective element for deoxidation, and may bepreferably added at a content ratio of 0.3˜1.0 weight % to a totalweight. When the amount of addition of silicon (Si) is less than 0.3weight %, a refining price may be increased. On the other hand, when theamount of addition of silicon (Si) exceeds 1.0 weight %, a material maybe hardened, degrading toughness and thus formability.

Manganese (Mn)

Manganese (Mn) is an essential element for increasing austenitic phasestability, and may be preferably added at a content ratio of 0.3˜2.0weight % to a total weight. When the amount of addition of manganese(Mn) is less than 0.3 weight %, a target physical property may bedifficult to secure. In contrast, when the amount of addition ofmanganese (Mn) exceeds 2.0 weight %, corrosion resistance may bedegraded due to excessive addition of manganese (Mn).

Chromium (Cr)

Chromium (Cr) is an essential element for improving corrosion resistanceand oxidation resistance in an operating environment of a fuel cell, andmay be preferably added at a content ratio of 15˜35 weight % to a totalweight. When the amount of addition of chromium (Cr) is less than 15weight %, it is difficult to secure proper oxidation resistance in theoperating environment of the fuel cell. In contrast, when the amount ofaddition of chromium (Cr) exceeds 35 weight %, a manufacturing cost maybe increased and toughness may be degraded, due to addition more thannecessary.

Copper (Cu)

Copper (Cu) is an economical additive element that may substitute forexpensive molybdenum (Mo) to increase pitting resistance in an acidicatmosphere where the fuel cell operates. However, when Cu is excessivelyadded, the performance of the fuel cell may be degraded due to elutionof Cu, and thus in consideration of this, in the present disclosure, Cuis limited to 1.0 weight % or less to the total weight of the stainlesssteel.

Nitrogen (N)

Nitrogen (N), which is an element for forming a nitride, existsinterstitially, such that when N is contained excessively, it may be anunfavorable to elongation rate and yield point elongation in spite ofincrease of strength. Thus, in the present disclosure, nitrogen may bepreferably limited to a content ratio of 0.01˜0.05 weight % to the totalweight of the stainless steel.

Titanium (Ti)

Titanium (Ti) is an effective element for forming C and N of the steelinto a carbonitride, but may degrade toughness when added excessively.Thus, in the present disclosure, in consideration of this, Ti is limitedto 0.3 weight % or less to the total weight of the stainless steel.

Niobium (Nb)

Niobium (Nb), like titanium, is an effective element for forming C and Nof the steel into a carbonitride, but may degrade toughness when addedexcessively. Thus, in the present disclosure, in consideration of this,Ti is limited to 0.3 weight % or less to the total weight of thestainless steel.

Phosphorus (P), Sulfur (S)

Phosphorus (P) reduces toughness as well as corrosion resistance, andthus in the present disclosure, is limited to 0.14 weight % or less tothe total weight of the stainless steel.

Sulfur (S) may form manganese sulfide (MnS) that becomes a startingpoint of corrosion and thus reduces corrosion resistance, and thus inthe present disclosure, in consideration of this, S is limited to 0.03weight % to the total weight of the stainless steel.

Hydrogen (H), Oxygen (O)

Hydrogen (H) and oxygen (O), which are inevitably added impurities, maybe preferably limited strictly to 0.004 weight % or less and 0.007weight % or less, respectively, to the total weight of the stainlesssteel of the present disclosure.

Hereinafter, with reference to the accompanying drawings, a method ofmanufacturing the stainless steel for the polymer fuel cell separatoraccording to an embodiment of the present disclosure will be described.

FIG. 1 is a flowchart illustrating a method of manufacturing stainlesssteel for a polymer fuel cell separator, according to an embodiment ofthe present disclosure.

As illustrated in FIG. 1, a method of manufacturing stainless steel fora polymer fuel cell separator, according to an embodiment of the presentdisclosure, may include stainless base material providing operationS110, degreasing processing operation S120, etching and desmutprocessing operation S130, and surface stabilization processingoperation S140. The method of manufacturing stainless steel for apolymer fuel cell separator according to an embodiment of the presentdisclosure may further include stabilization thermal-processingoperation S150. The stabilization thermal-processing operation S150 maynot be necessarily performed and may be omitted depending on needs.

Provide Stainless Base Material

In operation S110 of providing the stainless base material, thestainless base material is provided which includes, by weight %, C:0.01˜0.08%, Si: 0.3˜1.0%, Mn: 0.3˜2.0%, Cr: 15˜35%, Cu: 1.0% or less, N:0.01˜0.05%, Ti: 0.3% or less, Nb: 0.3% or less, and as the reset, Fe andother inevitable impurities, and has a passivation film formed on asurface thereof.

The stainless base material may further include one or more kinds of P:0.14 weight % or less, S: 0.03 weight % or less, H: 0.004 weight % orless, and O: 0.007 weight % or less.

In this case, the passivation film may cover the surface of thestainless base material. The passivation film may include Cr₂O₃. Thepassivation film may be formed on a top surface or a bottom surface ofthe stainless base material or both the top surface and the bottomsurface.

In this case, the stainless base material has an excellent thinningproperty and formability, but has a high electrical resistance due tothe Cr₂O₃ passivation film formed on the surface thereof, resulting inlow electrical conductivity.

Generally, the stainless base material may have, but not limited to, athickness of 0.05˜10 mm. The stainless base material may have a highcontact resistance of 200˜500 mΩ·cm² under a pressure of 1.0 MPa due tothe Cr₂O₃ passivation film generated on the surface thereof. Therefore,to use the stainless base material as the fuel cell separator, lowcontact resistance needs to be secured.

Degreasing Processing

In degreasing processing operation S120, the surface of the stainlessbase material may be degreased by being immersed in a degreasingsolution.

Herein, the degreasing processing may be performed to remove an organicmatter such as the Cr₂O₃ passivation film attached onto the surface ofthe stainless base material, etc. There is no significant distinctionbetween an acidic solution, a neutral solution, or an alkaline solutionin the degreasing processing, and any solution capable of removing anorganic matter adsorbed in the rolling process of the stainless basematerial may be used limitlessly.

More preferably, the degreasing solution may include 0.1˜3 mols of oneor more kinds of sodium hydroxide and potassium hydroxide, 0.01˜2 molsof silicate, 0.005˜0.8 mol of phosphate, 0.02˜3 mols of carbonate, asurfactant, and an emulsifier.

In this operation, the degreasing processing may preferably includeimmersion processing in the degreasing solution at 30˜70° C. for 0.5˜5minutes. When a degreasing processing temperature is lower than 30° C.or a degreasing processing time is shorter than 0.5 minute, there is ahigh risk of failing to completely remove the organic matter existing onthe surface of the stainless base material. On the other hand, when thedegreasing processing temperature is higher than 70° C. or thedegreasing processing time is longer than 5 minutes, the degreasingprocessing may become a factor that increases a manufacturing costwithout further effect increase, resulting in diseconomy.

Etching and Desmut Processing

In etching and desmut processing operation S130, after thedegreasing-processed stainless base material is etched with an etchingsolution, desmut processing may be performed with a desmut solution.

When etching processing is applied to remove heat and a natural oxidefilm existing on the surface of the stainless base material, a smut thatis a corrosion product may be generated excessively, and in this case,stains and discoloration may occur in the exterior, adversely affectinga contact resistance, corrosion resistance, and a contact angle. Thus,etching processing needs to be accompanied by desmut processing.

Due to the heat and the natural oxide film existing on the surface ofthe stainless base material, the interfacial contact resistance is equalto or greater than 300 mΩ·cm² under a pressure of 1.0 MPa, and thecontact angle is 75°˜100° with a hydrophobic property.

In this operation, etching processing may preferably include immersionfor 0.2˜2 minutes in an etching solution, heated to 40˜80° C., includingat least two or more of 2.5˜6.2 mols of sulfate ions, 0.1˜2.0 mols ofnitrate ions, and 1.0˜5.0 mols of fluorine.

Desmut processing may preferably include immersion for 0.5˜2 minutes ina desmut solution, heated to 40˜80° C., including 1.5˜6.0 mols ofhydrogen peroxide, 1.0˜4.0 mols of fluorine, and 0.001˜0.01 mol of acorrosion inhibitor. Herein, as the corrosion inhibitor, benzotriazolemay be used, without being limited thereto.

In this operation, when a thickness of 0.2˜5 μm is etched, an oxide filmexisting on the surface of the stainless base material may be completelyremoved and a hydrophilic property of a contact angle of 30°˜70° and acontact resistance of 10˜35 mΩ·cm² may be given. At a high concentrationand a high temperature of about 80° C., more etching may be made; at alow concertation or a low temperature, the oxide film may not beremoved, such that the contact resistance may exceed 30 mΩ·cm².

As such, when an excessive reaction occurs due to a processingcondition, a thickness of a product may significantly decrease due to anexcessive etching amount, failing to have characteristics as the fuelcell separator.

Surface Stabilization Processing

In surface stabilization processing operation S140, the etched anddesmut-processed stainless base material is subject to surfacestabilization processing with a surface stabilization solution.

The surface stabilization processing may be performed by immersing theetched and desmut-processed stainless base material at 50˜80° C. for0.5˜5 minutes, in the surface stabilization solution including 2.7˜13.8mols of nitrate, 0.05˜0.5 mol of sulfate, a surfactant, and astabilizer. Generally, in the case of well-known passivation processing,the oxide film may be densely formed, improving corrosion resistance,but increasing a contact resistance, and when the oxide film is thin andporous, corrosion resistance may become weak in spite of a low contactresistance.

On the other hand, when the surface stabilization processing isperformed like in the present disclosure, a surface modification layerof a thickness of several˜several tens of nanometers (nm) may be formedon the surface of the stainless base material, thereby giving superiorcorrosion resistance while maintaining a low contact resistance.

Through such surface stabilization processing, i.e., surfacemodification processing, stainless steel according to the presentdisclosure may have a surface modification layer having an interfacialcontact resistance of 10˜35 mΩ·cm² under a pressure of 1.0 MPa.

Stabilization Thermal Processing

In stabilization thermal processing operation S150, stabilizationthermal processing may be performed to stabilize the surface-stabilizedstainless base material.

Such stabilization thermal processing may be carried out to removemoisture remaining on the surface of the stainless base material afterthe surface stabilization processing and promote stabilization of thesurface modification layer.

To this end, the stabilization thermal processing may be preferablyperformed for 1˜10 minutes in an atmospheric condition of 150˜250° C.The surface modification layer formed by wet surface processing has ashelf life due to a continuous reaction with oxygen in the atmosphericenvironment, and to reduce surface property change with the shelf lifein the formation of the oxide layer, the activated surface modificationlayer is stabilized by thermal processing. Thus, at an environmenttemperature of 80° C. required in the fuel cell, a high-temperaturepotentiodynamic corrosion resistance of 15 μA/cm² (SCE) or less may beguaranteed.

The surface of the stainless steel for the polymer fuel cell separatormanufactured through the foregoing process (S110 through S150) ismodified by reconstructing the passivation film through surfacemodification processing, thereby securing superior interfacial contactresistance without being coated with a conductive material.

As such, in the present disclosure, by performing modificationprocessing of removing and reconstructing the passivation film on thesurface of the stainless base material, a low interfacial contactresistance, superior potentiodynamic corrosion resistance, and excellentsurface wettability for use as the fuel cell separator may beguaranteed.

As a result, the stainless steel for the polymer fuel cell separatoraccording to an embodiment of the present disclosure has an interfacialcontact resistance of 35 mΩ·cm² or less under a pressure of 1.0 MPawithout coating with a conductive material, thereby satisfyingproperties required for the polymer fuel cell separator.

EMBODIMENT

Hereinbelow, the structure and action of the present disclosure will bedescribed in more detail with reference to an embodiment of the presentdisclosure. However, this will be provided as a preferred example of thepresent disclosure, and is not interpreted as limiting the presentdisclosure in any sense.

A matter not described herein may be sufficiently technically construedby those of ordinary skill in the art and thus will not be described.

1. Manufacture Sample Comparative Example 1

A stainless base material including, by weight %, C: 0.05%, Si: 0.6%,Mn: 1.1%, Cr: 21%, Cu: 0.4%, N: 0.03%, Ti: 0.2%, Nb: 0.1%, and Fe as therest, and having a passivation film formed thereon is provided, and ismeasured without separate surface processing after alkaline degreasing.

Comparative Example 2

Au is coated on the stainless base material manufactured according toComparative Example 1 to a thickness of 1 μm to manufacture a sample.

Comparative Example 3

Surface modification of immersing the stainless base materialmanufactured according to Comparative Example 1 in a complex mixedsolution, heated to 30° C., of 4.7 mols of sulfuric acid, 1.1 mol ofnitric acid, and 2.1 mols of fluorine (F−) for 60 secs is performed tomanufacture a sample.

Comparative Example 4

After etching of immersing the stainless base material manufacturedaccording to Comparative Example 1 in a complex mixed solution, heatedto 30° C., of 4.7 mols of sulfuric acid, 1.1 mols of nitric acid, and2.1 mols of fluorine (F−), for 60 secs, desmut processing of immersionin a mixed solution, heated to 30° C., of 3.4 mols of hydrogen peroxideand 2.1 mols of fluorine (F−) for 60 secs is performed to manufacture asample.

Comparative Example 5

Except for immersion at an etching temperature of 60° C., a sample ismanufactured in the same manner as in Comparative Example 4.

Comparative Example 6

After etching of immersing the stainless base material manufacturedaccording to Comparative Example 1 in a complex mixed solution, heatedto 60° C., of 2.5 mols of sulfuric acid and 4.1 mols of fluorine (F−),for 60 secs, desmut processing of immersion in a mixed solution, heatedto 60° C., of 3.4 mols of hydrogen peroxide and 2.1 mols of fluorine(F−) for 60 secs is performed to manufacture a sample.

Comparative Example 7

After the same processing as Comparative Example 6, immersion in a mixedsolution, heated to 60° C., of 0.5 mol of sulfuric acid and 10.4 mols ofnitric acid for 100 secs is performed to manufacture a sample.

Comparative Example 8

Except for immersion in a complex mixed solution, heated to 60° C., of4.7 mols of sulfuric acid, 1.1 mols of nitric acid, and 2.1 mols offluorine (F−), for 150 secs, a sample is manufactured in the sameprocess as Comparative Example 7 after etching.

Comparative Example 9

Except for immersion in a complex mixed solution, heated to 60° C., of6.2 mols of sulfuric acid, 1.1 mols of nitric acid, and 2.1 mols offluorine (F−), for 150 secs, a sample is manufactured in the sameprocess as Comparative Example 7 after etching.

Embodiment 1

A stainless base material including, by weight %, C: 0.05%, Si: 0.6%,Mn: 1.1%, Cr: 21%, Cu: 0.4%, N: 0.03%, Ti: 0.2%, Nb: 0.1%, and as therest, Fe, and having a passivation film formed thereon is provided andalkaline degreasing is performed to remove an organic matter from thesurface.

Next, after etching is performed for 60 secs through immersion in acomplex mixed solution, heated to 60° C., of 4.7 mols of sulfuric acid,1.1 mols of nitric acid, and 2.1 mols of fluorine (F−), desmutting isperformed for 60 secs in a mixed solution, heated to 60° C., of 3.4 molsof hydrogen peroxide and 2.1 mols of fluorine (F−), after whichimmersion in a mixed solution, heated to 60° C., of 0.5 mol sulfuricacid and 11.4 mols nitric acid for 100 secs is performed to manufacturea sample.

Embodiment 2

A sample manufactured in the same manner as Embodiment 1 undergoesstabilization thermal processing at a temperature of 100° C. for 600secs to manufacture a sample.

Embodiment 3

A sample manufactured in the same manner as Embodiment 1 undergoesstabilization thermal processing at a temperature of 200° C. for 120secs to manufacture a sample.

Embodiment 4

A sample manufactured in the same manner as Embodiment 1 is subject tostabilization thermal processing at a temperature of 200° C. for 300secs to manufacture a sample.

Embodiment 5

Except for immersion in a complex mixed solution, heated to 60° C., of6.2 mols of sulfuric acid, 1.1 mols of nitric acid, and 2.1 mols offluorine (F−) and etching for 60 secs in the etching process, a sampleis manufactured in the same manner as Embodiment 1.

Embodiment 6

A sample manufactured in the same manner as Embodiment 5 is subject tostabilization thermal processing for 120 secs at 200° C. to manufacturea sample.

2. Evaluation of Physical Properties

Table 1 shows processing conditions for manufacturing samples accordingto Embodiments 1 through 6 and Comparative Examples 1 through 9, andTable 2 shows physical property evaluation results of the samplesmanufactured according to Embodiments 1 through 6 and ComparativeExamples 1 through 9.

FIG. 2 is a schematic diagram for describing a process of measuringinterfacial contact resistances of the samples according to Embodiments1 through 6 and Comparative Examples 1 through 9.

FIG. 3 is a graph showing potentiodynamic corrosion resistancemeasurement results of the samples manufactured according to ComparativeExamples 1 through 3 and 5, and FIG. 4 is a graph showingpotentiodynamic corrosion resistance measurement results of the samplesmanufactured according to Embodiments 1 through 4.

FIG. 5 shows pictures of contact angles of the samples manufacturedaccording to Comparative Examples 1 through 3 and 5, and FIG. 6 showspictures of contact angles of the samples manufactured according toEmbodiments 1 through 4.

FIG. 7 shows pictures indicating whether a corrosion product (smut)remains for samples manufactured according to Embodiments 3 and 5 andComparative Example 3.

FIG. 8 is a graph showing XPS analysis results of the samplesmanufactured according to Embodiments 1 and 3 and Comparative Example 1,and FIG. 9 is a graph showing results of evaluating stack performance ofunit cells using the samples manufactured according to Embodiments 3 and6 and Comparative Example 2.

1) Surface Cleanliness

Surface cleanliness is observed by naked eyes, and in the case ofdiscoloration due to residual corrosion products on the surface andemergence of foreign substances when wiped with a cotton swab, etc.,such presence is indicated by X, and absence is indicated by O.

2) Pitting Stability

As a criterion for evaluating official stability, presence of pitting of1 μm or more after surface modification is indicated by X and absence ofpitting and erosion on the surface is indicated by O. Herein, whenerosion and pitting occur on a stainless steel sample after surfacemodification, corrosion resistance may be degraded in a fuel celloperating environment, such that whether erosion and pitting occur aftersurface modification is important.

3) Interfacial Contact Resistance

As shown in FIG. 2, by using a method of measuring current under apressure of 1.0 MPa after insertion of stainless steel (sample) betweengas diffusion layers (GDLs), a contact resistance after surfacemodification is measured.

4) Corrosion Current

For corrosion current, Tafel slope evaluation of a potentiostat is used,and corrosion current upon application of a voltage of 0.6V is measuredby performing potentiodynamic evaluation in a mixed solution, heated to80° C., of 0.1 N of sulfuric acid and 2 ppm of hydrofluoric acid.

5) Contact Angle

For a contact angle, equipment of a model DM700 of Japan's KYOWA companyis used, and the contact angle is measured after distilled water of 3 μlis dropped on the surfaces of the samples. Herein, when the contactangle on the surface of the stainless steel is high after surfacemodification, water discharge is reduced due to the flooding phenomenoncaused by water condensed on the surface of the separator under the fuelcell operating environment, such that the non-uniform flow of thereactive gas and degradation of diffusion of the reactive gas cause adeficiency of the reactive gas in the electrode, deteriorating theperformance of the fuel cell, and therefore, it is important toimplement a hydrophilic surface having a low contact angle.

6) Thickness Reduction

For thickness reduction after surface modification, a thickness of asample is measured before and after surface modification by using amicrometer. When a thickness of a material is reduced due to anexcessive etching reaction, an engaging pressure of about 1000 stackedseparators or more is lowered, degrading the performance of a fuel cell,such that it is important to implement stable surface modificationwithout thickness reduction.

7) Stack Evaluation

For separator performance evaluation, separator/GDL/MEA/GDL/separatorare stacked in that order to form a unit cell. As the separator, therelated art and Embodiments 3 and 6 are used, and performance comparisonis performed in the same cell.

TABLE 1 Stabilization Thermal Etching Desmut Surface StabilizationProcessing sulfuric fluoride temper- hydrogen fluoride temper- sulfurictemper- temper- acid nitrogen (F) time ature peroxide (F) time atureacid nitrogen time ature time ature Category (mol/L) (mol/L) (mol/L)(sec) (° C.) (mol/L) (mol/L) (sec) (° C.) (mol/L) (mol/L) (sec) (° C.)(sec) (° C.) Comparative — — — — — — — — — — — — — — — Example 1Comparative — — — — — — — — — — — — — — — Example 2 Comparative 4.7 1.12.1 60 30 — — — — — — — — — Example 3 Comparative 4.7 1.1 2.1 60 30 3.42.1 60 30 — — — — — Example 4 Comparative 4.7 1.1 2.1 60 60 3.4 2.1 6030 — — — — — Example 5 Comparative 2.5 — 4.1 60 60 3.4 2.1 60 60 — — — —— Example 6 Comparative 2.5 — 4.1 60 60 3.4 2.1 60 60 0.5 10.4 100 60 —— Example 7 Comparative 4.7 1.1 2.1 150 60 3.4 2.1 60 60 0.5 11.4 100 60— — Example 8 Comparative 6.2 1.1 2.1 150 60 3.4 2.1 60 60 0.5 11.4 10060 — — Example 9 Embodiment 4.7 1.1 2.1 60 60 3.4 2.1 60 60 0.5 11.4 10060 — — 1 Embodiment 4.7 1.1 2.1 60 60 3.4 2.1 60 60 0.5 11.4 100 60 600100 2 Embodiment 4.7 1.1 2.1 60 60 3.4 2.1 60 60 0.5 11.4 100 60 120 2003 Embodiment 4.7 1.1 2.1 60 60 3.4 2.1 60 60 0.5 11.4 100 60 300 200 4Embodiment 6.2 1.1 2.1 60 60 3.4 2.1 60 60 0.5 11.4 100 60 — — 5Embodiment 6.2 1.1 2.1 60 60 3.4 2.1 60 60 0.5 11.4 100 60 120 200 6

TABLE 2 Contact Corrosion Contact Thickness Surface Pitting ResistanceCurrent Angle Reduction Category Cleanliness Stability (mΩ · cm²)(μA/cm²) (°) (μm) Comparative ∘ ∘ 400 7 82 — Example 1 Comparative ∘ ∘15 1 76 — Example 2 Comparative x ∘ 250 25 125 — Example 3 Comparative x∘ 35 30 80 — Example 4 Comparative x ∘ 25 19 118 — Example 5 Comparative∘ x 23 17 75 — Example 6 Comparative ∘ x 24 10 75 — Example 7Comparative ∘ ∘ 10 16 30  5 Example 8 Comparative ∘ ∘ 13 12 23 10Example 9 Embodiment 1 ∘ ∘ 10 14 19 — Embodiment 2 ∘ ∘ 12 10 32 —Embodiment 3 ∘ ∘ 12 5 50 — Embodiment 4 ∘ ∘ 13 1 55 — Embodiment 5 ∘ ∘13 12 23 — Embodiment 6 ∘ ∘ 19 5 57 —

As shown in Tables 1 and 2 and FIGS. 2 through 7, the samples accordingto Embodiments 1 through 6 obtain clean surfaces without foreignsubstances thereon, obtain stable surfaces without pitting occurringthereon, and obtain stable surfaces without erosion occurring thereon(without thickness change).

Moreover, the samples according to Embodiments 1 through 6 obtain aninterfacial contact resistance of 20 mΩ·cm² or less under a pressure of1.0 MPa and show a corrosion current of 15 μA(SCE) or less at a voltageof 0.6V in a solution, heated to 60° C., of 0.1 N of sulfuric acid and 2ppm of hydrofluoric acid, simulating a harsh operating environment ofthe polymer fuel cell.

On the other hand, in the case of the sample according to ComparativeExample 3, pitting and erosion are not observed, but the sample has thenon-uniform etching surface and the corrosion product remaining on thesurface, having a bad effect on contact resistance, corrosion current,and contact angle. As shown in FIG. 5, on the sample according toComparative Example 3, the corrosion product remains.

In the case of the sample according to Comparative Example 4, desmutprocessing for removing the corrosion product generated in the sampleaccording to Comparative Example 3 is performed to improve a contactresistance, but residual foreign substances are not completely removedfrom the surface.

In the case of the sample according to Comparative Example 5, as aresult of etching processing at a temperature of 60° C., a lot ofcorrosion product is generated on the surface in comparison to in thesample according to Comparative Example 4, and the residual foreignsubstances are not completely removed from the surface in spite ofdesmut processing.

For the sample according to Comparative Example 6, pitting occurs on thesurface when nitric acid is not added to a complex mixed solution. It isobserved that when the nitric acid is added, occurrence of pitting islowered by induction of a full reaction.

For the sample according to Comparative Example 7, corrosion resistanceis slightly improved by surface stabilization. It is observed thatexposure to a nitric acid solution of a high concentration leads toimprovement of surface passivation performance.

For the samples according to Comparative Example 8 and ComparativeExample 9, exposure to an etching solution for a long time results in anexcessive amount of a corrosion product and thus reduction in thethickness of a material surface on both sides. As the thickness of thematerial decreases, the engaging pressure of the 1000 stacked separatorsor more is lowered, degrading fuel cell performance, and desired surfacecharacteristics are not obtained due to excessive etching.

As in Embodiment 1 and Embodiment 2, according to the method proposed inthe present disclosure, stainless steel for a fuel cell separator may bemanufactured through wet surface modification, which satisfiesinterfacial contact resistance, corrosion resistance, and contact anglecharacteristics required in the polymer fuel cell, and it is observedthat corrosion resistance is improved through further thermalprocessing.

However, in Embodiment 2, due to an inefficient processing time in acontinuous manufacturing process, a processing time needs to beimproved.

In Embodiment 3 and Embodiment 4, a processing temperature is increasedwhen compared to in Embodiment 2, thereby innovatively reducing aprocessing time.

However, when the temperature is 300° C. or higher, an excessive oxidelayer is formed on the surface, degrading contact resistancecharacteristics, such that the temperature needs to be controlled below300° C.

Comparing Embodiment 3 with Embodiment 6, it is observed that aninterfacial contact resistance increases after thermal processing whenthe concentration of sulfuric acid is high in the etching process. It isdetermined that when the surface unstable due to the excessive amount ofetching in the case of exposure to the high sulfuric acid concentrationis subject to thermal processing, an oxide layer is generated a lot,increasing contact resistance.

Meanwhile, as shown in FIG. 8, by comparing Comparative Example 1 withEmbodiment 1, it is observed that a content of oxygen is reduced byabout 15% and a content of chromium is increased by about 15% in asurface top layer after wet surface processing. It is seen from thisthat as the oxide layer of the surface is removed and the natural oxidefilm is re-generated in etching processing, a content of a chromiumoxide on the surface is increased.

Moreover, by comparing Embodiment 1 with Embodiment 3, it is seen that acontent of Cr is reduced by about 15% after wet surface processing, andit is thought that iron is oxidized in thermal processing and thus acomposition ratio on the surface is increased. In this way, it isdetermined that in a process of removing and reconstructing an oxidelayer, which is a factor hindering an interfacial contact resistance,from the surface, using the method proposed in the present disclosure,the surface is modified, thus improving a contact resistance.

In the foregoing test, in a method of modifying the surface of thestainless steel having a composition range according to the presentdisclosure, for etching processing in a complex mixed solution ofsulfuric acid, nitric acid, and fluorine, desmut processing in a mixedsolution of hydrogen peroxide and fluorine, and surface stabilizationprocessing in a mixed solution of sulfuric acid and nitric acid,conditions such as a temperature, composition, etc., of the solutionserve as an important factor for implementation of interfacial contactresistance, corrosion resistance, and contact angle characteristics.

Thus, by modifying the passivation film of the stainless base material,low contact resistance, low corrosion current, and low contact angle maybe secured, and by modifying a passivation layer through control of acomposition ratio of Cr, Fe, and 0 elements, stainless steel proper forthe polymer fuel cell separator may be produced.

Meanwhile, as shown in FIG. 9, performance evaluation results are shownwith respect to unit cells using stainless steel according to therelated art (Comparative Example 2) and Embodiment 3 and Embodiment 6according to the present disclosure.

As can be seen from the stack evaluation results of FIG. 9, there islittle difference in performance between unit cells using the sampleaccording to Comparative Example 2 (the related art) in which Ag iscoated on the stainless base material to a thickness of 1 μm and thesamples according to Embodiment 3 and Embodiment 6 of the presentdisclosure.

While the embodiments of the present disclosure have been mainlydescribed so far, various changes or modifications can be made at thelevel of those of ordinary skill in the art. Such changes andmodifications can be understood as falling within the present disclosurewithout departing from the technical spirit of the present disclosure.Accordingly, the scope of the present disclosure should be determined bythe appended claims.

EXPLANATION OF NUMERAL REFERENCES

-   -   S101: Stainless Base Material Providing Operation    -   S120: Degreasing Processing Operation    -   S130: Etching and Desmut Processing Operation    -   S140: Surface Stabilization Processing Operation    -   S150: Stabilization Thermal Processing Operation

1. Stainless steel for a polymer fuel cell separator, the stainlesssteel comprising: by weight %, C: 0.01˜0.08%, Si: 1.0% or less, Mn: 2.0%or less, Cr: 15˜35%, Cu: 1.0% or less, N: 0.05% or less, Ti: 0.3% orless, Nb: 0.3% or less, Fe, and other inevitable impurities; and asurface modification layer having an interfacial contact resistance of10˜35 mΩ·cm² through surface modification processing, wherein apotentiodynamic corrosion resistance is 1.0˜15.0 μA/cm² and a contactangle is 30˜70°.
 2. The stainless steel of claim 1, further comprisingone or more kinds of P: 0.14 weight % or less, S: 0.03 weight % or less,H: 0.004 weight % or less, and O: 0.007 weight % or less.
 3. Thestainless steel of claim 1, wherein the surface modification processingcomprises degreasing processing of performing immersion in a degreasingsolution, etching and desmut processing of performing etching with anetching solution and desmut processing in a desmut solution, and surfacestabilization processing of performing immersion in a surfacestabilization solution.
 4. A method of manufacturing stainless steel fora polymer fuel cell separator, the method comprising: (a) providing astainless base material comprising, by weight %, C: 0.01˜0.08%, Si: 1.0%or less, Mn: 2.0% or less, Cr: 15˜35%, Cu: 1.0% or less, N: 0.05% orless, Ti: 0.3% or less, Nb: 0.3% or less, and as the rest, Fe and otherinevitable impurities, the stainless base material having a passivationfilm formed on a surface thereof; (b) performing degreasing processingby immersing the surface of the stainless base material in a degreasingsolution; (c) after etching the degreasing-processed stainless basematerial with an etching solution, performing desmut processing with adesmut solution; and (d) performing surface stabilization on the etchedand desmut-processed stainless base material with a surfacestabilization solution, wherein after (d), the stainless steel comprisesa surface modification layer having an interfacial contact resistance of10˜35 mΩ·cm² through surface modification processing.
 5. The method ofclaim 4, wherein the stainless base material further comprises one ormore kinds of P: 0.14 weight % or less, S: 0.03 weight % or less, H:0.004 weight % or less, and O: 0.007 weight % or less.
 6. The method ofclaim 4, wherein in (b), the degreasing processing comprises performingimmersion processing in the degreasing solution at 30˜70° C. for 0.5˜5minutes.
 7. The method of claim 4, wherein in (c), the etchingprocessing comprises performing immersion for 0.2˜2 minutes in anetching solution, heated to 40˜80° C., comprising at least two or moreof 2.5˜6.2 mols of sulfate ions, 0.1˜2.0 mols of nitrate ions, and1.0˜5.0 mols of fluorine, and the desmut processing comprises performingimmersion for 0.5˜2 minutes in a desmut solution, heated to 40˜80° C.,comprising 1.5˜6.0 mols of hydrogen peroxide, 1.0˜4.0 mols of fluorine,and 0.001˜0.01 mol of a corrosion inhibitor.
 8. The method of claim 4,further comprising, after (d), (e) performing stabilization thermalprocessing for stabilization of the surface-stabilized stainless basematerial.
 9. The method of claim 4, wherein the stabilization thermalprocessing is performed at 150˜250° C. for 1˜10 minutes.